In recent years, application of reproducible natural fibers has been drawing attention as a substitute for oil resources and in response to increasing environmental awareness. Of all natural fibers, cellulose fibers, in particular, those derived from wood (pulp) are widely used, mainly as paper products. The widths of cellulose fibers used for paper are 10 to 50 μm in most cases. Paper (sheet) obtained from such cellulose fibers is opaque, and is widely used as printing paper. Meanwhile, by treating (refining or grounding) cellulose fibers using a refiner, a kneader, a sand grinder or the like so as to convert them into finer pieces (microfibrils), transparent paper (e.g., glassine) can be obtained. Conventionally, nonwoven fabrics made from cellulose fibers have been used as sheets for printing paper, books, and the like with the addition of a sizing agent, a paper strength additive, and the like. Further, nonwoven fabrics that are made from cellulose fibers with the use of permeability to gas, liquid, or the like are considered to be used for filters, capacitor elements, separators of batteries or capacitors, and the like.
Patent Literature 1 discloses a composite of (a) a nonwoven fabric containing cellulose and (b) resin other than cellulose, in which the content of the (a) component is greater than or equal to 0.1 weight % and less than or equal to 99 weight %, and the content of the (b) component is greater than or equal to 1 weight % and less than or equal to 99.9 weight %. In Patent Literature 1, pores of the (a) nonwoven fabric containing cellulose are filled with the (b) resin other than cellulose.
Patent Literature 2 describes a plastic film with a hard coat layer that is obtained by laminating a primer layer and a hard coat layer on a transparent plastic film. Resin that forms the hard coat layer is a hardened resin composition that contains pentafunctional or higher functional (meth)acrylate, monofunctional (meth)acrylate, and a photopolymerization initiator as the essential components. The transparent plastic used in Patent Literature 2 is a polyester film, in particular, a PET film.
Patent Literature 3 discloses a nonwoven fabric which was obtained by papermaking from cellulose fibers with an average fiber diameter of 0.1 to 20 μm and cellulose nanofibers with an average fiber diameter of less than 100 nm, and a separator for a storage battery element that is made from such a nonwoven fabric.